Various articles, such as aviation and aerospace vehicles, are often coated to achieve properties such as solvent resistance, fuel and hydraulic fluid resistance, weather resistance, abrasion resistance, hardness, and/or aesthetics. To that end, polyurethane coatings have been used which generally include activators, base components and thinners. Typically, the activator is an organic polyisocyanate, the base component is a hydroxyl-containing polymeric resin, and the thinner is a solvent mixture. To achieve high performance, which is required for aerospace applications by way of example, large amounts of solvent are used to balance film properties with appearance and aesthetics. The use of such large amounts of solvent, however, yields large amounts of volatile organic components (“VOC”'s), i.e. about 420 g/L in a typical solvent-borne formulation according to the Environmental Protection Agency's calculation method. Reductions in the amount of VOCs used in these formulations are desirable for ecological and economic reasons, as well as to comply with ever-changing governmental standards. Therefore, efforts have been made to replace the organic solvents in solvent-borne polyurethane coating compositions with water.
To reduce the amount of VOCs in polyurethane coatings, water-borne polyurethane coatings have been developed. However, thus far, water-borne polyurethane coatings have been unable to match the high performance of solvent-borne coatings. The performance of water-borne polyurethane coatings suffers because the water-borne polyol resin used in the coatings is dispersed in water for storage prior to combination with the activator. The water-borne polyol resin often has low molecular weight and many ester linkages, making it susceptible to hydrolysis over time. Hydrolysis decreases the overall molecular weight of the resin, yielding lower molecular weight products exhibiting poor impact resistance, pot-life, gloss, and the like. In addition, the hydrolysis rate is difficult to control under different conditions such as batch number, pH, and storage time, resulting in significant variations in film performance.
Also, water-borne polyurethane coatings are often prepared by high shear mixing a water-borne polyol resin with a hydrophilic isocyanate. High shearing energy is needed to intimately mix the hydrophilic isocyanate with the polyol resin. To overcome the barrier between the polyol resin colloid and the isocyanate, high shear energy is used to facilitate the migration of the isocyanate into a micelle of the polyol resin. The curing reaction thus occurs inside the new micelle to form the water-borne polyurethane coating composition. However, high shear mixing uses equipment, such as dissolver mixers and jet dispersing spray guns, that require high pressure and high shearing energy to intimately mix the isocyanate and polyol components. This equipment is not available for many applications; for example, the equipment is not available for fast field repair of, for example, automobiles as well as aviation and aerospace vehicles.
Finally, the polyol components of conventional water-borne polyurethane coatings include dispersions of polyols in water. As discussed above, such dispersions produce unstable polyol components because the polyols may hydrolyze into small molecules. Also, dispersion of the polyols in water makes high shear mixing necessary. In addition, polyol components often include pigments and other additives, such as aluminum powder, that are not stable in aqueous phases under storage conditions. These pigments and additives may react with water, limiting the development of the water-borne coating and adversely affecting the performance of the coating.